There are known toothed transmission belts comprising a body made from elastomeric material having teeth on one of the surfaces and a fabric adhering to the surface of the teeth.
During the process of manufacturing the belt, the fabric is made to adhere to the teeth of the belt. Initially, therefore, the length of the fabric must be substantially equal to the circumference of the belt, whereas when the teeth are formed the fabric must have a length equal to the profile of the teeth of the toothed belt. It is therefore necessary for this fabric to be easily extensible in the longitudinal direction of the toothed belt.
For this reason there is a known method in which the threads of the fabric extending in the longitudinal direction of the belt are made from elastic material, for example the threads may consist of nylon as described in European Patent EP-271587.
In the following text, these threads of the fabric which extend in the longitudinal direction of the belt will be called the weft of the fabric, while the threads which extend in the transverse direction of the belt will be called the warp.
Recently, the use of toothed belts in engines for motor vehicles has required the reformulation of the composition of the threads of the fabric so that they can withstand higher temperatures and have increased strength. However, it is generally difficult to process fibres and obtain threads which have characteristics of high strength and heat resistance and which are also elastic. The use of composite fibres, or fibres composed of different materials, has therefore been proposed. Japanese patent application JP-63-15628 describes the use of a composite thread in which an aramid thread forms a spiral covering of an elastic polyurethane thread used in the weft of the fabric of a toothed belt. When the elastic polyurethane thread is not stretched, the spiral-wound aramid thread is slackened in such a way that the elastic thread can be elongated. The result is that the aramid covering thread can form loops around the elastic thread. When the covering thread forms the weft of the fabric of the toothed belt, the loops extend towards the exterior of the fabric, coming directly into contact with the teeth of the pulley. Thus the strength of the fabric is decreased and consequently the durability of the toothed belt is also decreased.
Moreover, in the composite thread described above the high-strength aramid thread is exposed on the outer surface of the fabric. It is thus substantially incorporated in the elastomeric material forming the belt. If the belt is subjected to repeated deformations, the aramid thread is also subjected to repeated bending and stretching which diminish its strength and reduce its durability.
EP-637704 describes a toothed belt comprising a fabric in which the longitudinal threads, which extend in the longitudinal direction of the toothed belt, are formed from covered threads, each of which is prepared by winding a thread with high strength and heat resistance on a core of elastic thread and then winding a further thread of synthetic fibre on the preceding ones.
From JP 05-118390, a method is also known for manufacturing a fabric for toothed belts consisting of a weft and a warp, in which the weft consists of an elastic thread covered with a composite thread formed by a covering thread which is wound round a thread with high heat resistance and high strength, and this structure will be defined in what follows as single-wound.
However, this solution does not provide sufficient assurance of strength and durability of the belt and does not completely prevent the formation of loops of the thread with high strength and heat resistance which may be formed in the interstices formed by the covering thread.
Moreover, it would be useful to be able to achieve a further increase in abrasion resistance, the adhesion between the fabric and the body of the belt and, in particular, in the breaking load which the fabric can withstand.